1. Field of the Invention
The present invention generally relates to electron-beam-exposure methods and electron-beam-exposure devices, and particularly relates to an electron-beam exposure method and an electron-beam exposure device which carry out a block exposure by using a block mask.
2. Description of the Related Art
In recent years, electron-beam-exposure methods and electron-beam-exposure devices have been used in the field in order to increase circuit density of integrated circuits. In the electron-beam-exposure scheme, the size of an electron beam can be made as small as several angstroms, so that fine patterns smaller than 1 .mu.m can be created.
If a pattern is to be drawn by an electron beam with a single stroke, an exposure process will take an undesirably long time. To counter this, a block-exposure method has been proposed, in which a plurality of blocks, each having at least one aperture of various pattern shapes, are provided on an aperture mask, and an electron beam is directed to a selected one of these blocks. Here, the aperture mask is made from a plate capable of blocking an electron beam, so that an electron beam passing through an aperture of the mask has a cross-sectional shape corresponding to a shape of the aperture. The electron beam passing through the apertures of the selected block is then directed to an object such as a wafer to form a pattern of the apertures. This block-exposure method can achieve as high a throughput as 1 cm.sup.2 /sec, and is known to have superior characteristics in terms of fineness of the processing, the positioning accuracy, the turn-around speed, and reliability.
FIG. 1 is a block diagram of an example of an electron-beam-exposure device of the block-exposure type of the prior art.
In FIG. 1, an electron-beam exposure device 100 includes an exposure-column unit 110 and a control unit 150. The exposure-column unit 110 includes an electron-beam generator 114 having a cathode 111, a grid 112, and an anode 113. The exposure-column unit 110 further includes a first slit 115 for rectangular shaping of the electron beam, a first lens 116 converging the shaped beam, and a slit deflector 117 deflecting a position of the shaped beam on a mask 120 based on a deflection signal S1. The exposure-column unit 110 further includes second and third lenses 118 and 119 opposing each other, the mask 120 mounted movably in a horizontal direction between the second and third lenses 118 and 119, and first-to-fourth deflectors 121 through 124 deflecting the beam between the second and third lenses 118 and 119 based on position information P1 through P4 to select one of a plurality of holes (apertures) provided through the mask 120. The exposure-column unit 110 further includes a blanking aperture electrode 125 cutting off or passing the beam according to a blanking signal, a fourth lens 126 converging the beam, an aperture 127, a refocus coil 128, and a fifth lens 129. The exposure-column unit 110 further includes a dynamic focus coil 130, a dynamic stigmator coil 131, an objective lens 132 projecting the beam on to a wafer, and a main deflector 133 and a sub deflector 134 positioning the beam on the wafer according to exposure-position signals S2 and S3. The exposure-column unit 110 further includes a stage 135 carrying the wafer to move it in X-Y directions, and first-to-fourth alignment coils.
The control unit 150 includes a memory media 151 comprising a disk or MT recorder for storing design data of integrated circuits, and a CPU 152 controlling the electron-beam exposure device. The control unit 150 further includes a data-management unit 153, an exposure-management unit 159, a mask-stage controlling unit 160, a main-deflector-deflection setting unit 161, and a stage controlling unit 162, all of which are connected via a data bus (i.e., VME bus). Exposure data includes main-deflector data and sub-deflector data, and is stored in a buffer memory 154 via the data-management unit 153 prior to the exposure process. The buffer memory 154 is used as a high-speed buffer for reading the exposure data, thereby negating influence of low-speed data reading from the memory media 151.
The main-deflector data is set in the main-deflector-deflection setting unit 161 via the exposure-management unit 159. The exposure-position signal S2 is output after the deflection amount is calculated, and is provided to the main deflector 133 via the DAC/AMP 170. Then, the sub-deflection data for exposing a selected field is read from the data-management unit 153, and is sent to the sub-deflector-deflection setting unit 155. In the sub-deflector-deflection setting unit 155, the sub-deflection data is broken down into shot data by the pattern generating unit 156, and is corrected by the pattern-correction unit 157. These circuits operates in a pipeline according to a clock signal generated by the clock setting unit 158.
After the processing of the pattern-correction unit 157, a signal S1 for setting a slit size, mask-deflection signals P1 through P4 for determining a deflected position on the mask 120 of the beam deflected according to the signal S1 after passing through the first slit 115, a signal S3 for determining a position on the wafer of the beam shaped by the mask 120, and a signal S4 for correcting distortion and blurring of the beam are obtained. The signal S1, the mask-deflection signals P1 through P4, the signal S3, and the signal S4 are supplied to the exposure-column unit 110 via the DAC(digital-to-analog converter)/AMP(amplifier) 166, DAC/AMP 167, DAC/AMP 171, and DAC/AMP 169. Also, the clock setting unit 158 provides a blanking controlling unit 165 with a B signal. A BLK signal for controlling the blanking operation from the blanking controlling unit 165 is supplied to the blanking aperture electrode 125 via the AMP 168.
An exposure position on the wafer is controlled by the stage controlling unit 162. In doing so, a coordinate position detected by a laser interferometer 163 is supplied to the stage controlling unit 162. Referencing to the coordinate position, the stage controlling unit 162 moves the stage 135 by driving a motor 164.
In this manner, the control unit 150 controls the exposure-column unit 110 so that the electron beam emitted from the electron-beam generator 114 is shaped rectangularly by the first slit 115, converged by the lenses 116 and 118, deflected by the mask deflectors 121 and 122, and directed to the mask 120. The electron beam having passed through the mask 120 passes through the blanking aperture electrode 125, is converged by the fourth lens 126, is deflected to a center of a sub field of about 100-.mu.m square by the main deflector 133, and is deflected within this sub field by the sub deflector 134.
In general, electron-beam-exposure methods have a problem of Coulomb interaction. This is a phenomenon in which electrons of the electron beam are repelled by each other so that a cross section of the electron beam is blurred in general proportion to the current amount of the beam. Especially, at the focus point of the electron beam, a probability of electrons interacting with each other is increased to bring about undesirable blurring of the image.
In the block-exposure method using an electron beam passing through the apertures to form a fine pattern over a wide area, the current amount of the electron beam tends to be large, making the block-exposure method susceptible to the Coulomb interaction. Use of a shorter-focusing-distance lens can lessen the effect of the Coulomb interaction, but not to a sufficient extent.
In order to reduce the current amount of the electron beam in the block-exposure method, two methods can be used. These two methods are 1) reducing the current density of the electron beam and 2) reducing the size of an exposed area (size of apertures). Since the current amount is provided as a product of the current density and the exposure-area size, a reduction in one of these two factors can lower the current amount.
When the current density is lowered as in method 1), exposure time must be increased in order to sustain a required exposure amount of the wafer. Thus, throughput, i.e., production is sacrificed. To keep a throughput reduction as small as possible, it is desirable to change the current density according to an area size of the apertures. Namely, an exposure with a low current density is carried out in a long time when the area size of the apertures is large, and an exposure with a high current density is carried out by taking a short period of time when the area size of the apertures is small. In this case, however, the current density is changed during the exposure process, leading to an instability in the operation of the device. Because of this reason, this method is not practical.
In order to reduce the area size of the apertures in the method 2), a block size (area size selected for one shot of an electron beam) may be made smaller when a pattern density is high, and may be made larger when the pattern density is small. By doing so, it is possible to avoid a sacrifice of the throughput. In this case, however, it is likely that some portions in a block size may not be used effectively. Also, there is a downside in that the block extraction process and the block-exposure process become complicated.
Accordingly, an exposure method using a low current density while sacrificing the throughput is used in the prior-art block-exposure process.
There is also another problem in the block-exposure method, and this problem will be described below.
As described above, in the electron-beam-exposure device using the block-exposure method, a plurality of blocks each having at least one aperture of various pattern shapes are provided on an aperture mask, and an electron beam is directed to a selected one of these blocks. The electron beam passing through the apertures of the selected block is then directed to a wafer to form a pattern of the apertures on the wafer.
Each block has apertures of a different pattern shape, and the current amount of the electron beam passing through the apertures of a given block is dependent on the area size of the apertures. Thus, when different blocks are selected, different amounts of current are directed via the electron beam to the wafer.
FIGS. 2A and 2B are illustrative drawings showing examples of blocks having different aperture area sizes.
Compared to a pattern of apertures shown in FIG. 2A, a pattern of apertures of FIG. 2B has a smaller area size. When these two patterns are exposed at the same exposure amount, an appropriate exposure amount for the pattern of FIG. 2A will result in an under-exposure for the pattern of FIG. 2B, and an appropriate exposure amount for the pattern of FIG. 2B will result in an over-exposure for the pattern of FIG. 2A. Namely, use of the same exposure amount can not properly draw both patterns of FIG. 2A and FIG. 2B. Thus, an exposure amount is generally increased for a pattern having a smaller aperture area size as shown in FIG. 2B, compared to when a pattern having a larger aperture area size as shown in FIG. 2A is exposed. In order to increase the exposure amount, either the exposure time can be increased or the current density of the electron beam can be increased, as previously described.
As described above, different exposure amounts may be set for different patterns having different aperture area sizes. However, there is a problem in this method concerning an adjustment of the exposure amount.
FIG. 3 is an illustrative drawing showing a block and an aperture pattern within the block for explaining the problem concerning the exposure amount adjustment.
In FIG. 3, a block 200 includes an aperture 201 and an aperture 202. Denoting an area size of the aperture 201 as A1 and an area size of the aperture 202 as A2, A1 is much smaller than A2. In the exposure-amount-adjustment method, an exposure amount for a block is determined according to a total area size of apertures of that block. Therefore, the exposure amount for the block 200 is determined based on a total area size A1+A2. When an exposure amount is determined in this manner, an exposure of the aperture 202 is properly conducted. However, the aperture 201 is under-exposed, so that an appropriate pattern is not created. Namely, an aperture having a smaller area size tends to be under-exposed when a plurality of apertures having vastly different area sizes are included in a block. This is because the exposure amount is adjusted block by block.
In addition, there is the influence of the Coulomb interaction described above. In order to suppress the Coulomb interaction, the current density of the electron beam needs to be reduced while an exposure time is increased to keep an appropriate exposure amount, as previously described. However, it is not desirable to reduce the current density indiscriminately for all the blocks including blocks free from the Coulomb interaction, because it results in a lower throughput. Also, setting a different current density for each different block leads to inordinate changes in the current density, thereby making the operation of the device unstable.
Accordingly, there is a need for a block-exposure method and a block-exposure device which can reduce an influence of the Coulomb interaction without lowering the throughput, without making the device operation unstable, and without complicating the processes, and, also, there is a need for a mask and a method of creating the mask used in such a block-exposure method and such a block-exposure device.
Also, there is a need for a block-exposure method and a block-exposure device which can reduce an influence of the Coulomb interaction and can create a fine pattern of a small-size aperture with an appropriate exposure amount.